There are a plurality of known methods for character recognition, especially for recognition of handwritten characters, which requires especially good interpretation of the character. Several of the known methods are based on the detection of each stroke of the pen when a handwritten character is being formed. Geometric characteristics, such as directions, inclinations and angles of each stroke or part of a stroke, are determined and compared to corresponding data for stored, known characters. The written character is supposed to be the stored character whose geometric characteristics best correspond to the geometric characteristics of the written character. The geometric characteristics are related to an xy-coordinate system, which covers the used writing surface. Such known methods are disclosed in, for instance, U.S. Pat. No. 5,481,625 and U.S. Pat. No. 5,710,916. A problem in such methods is that they are sensitive to rotation. For example, if one writes diagonally over the writing surface, the method has difficult ties in correctly determining what characters are being written.
U.S. Pat. No. 5,537,489 discloses a method for preprocessing the characters by normalizing them. The written character is sampled, and each sample is represented as a pair of coordinates. Instead of solely comparing the characters in the coordinate plane, the transformation is determined which best adjusts the written character to a model character. Indirectly, also rotation and certain types of deformations, which the above-mentioned methods cannot handle, are thus taken into account. The transformation is used to normalize the written character. In particular, the character is normalized by being translated so that its central point is in the origin of coordinates, where also the central point of the model character is found, after which the character is scaled and rotated in such a manner that it corresponds to the model character in the best possible way. A disadvantage of this method is that the normalization requires computing power and that in any case the choice of model characters has to take place by determining what model character the written character resembles the most.
Another method which certainly can handle rotations is disclosed in U.S. Pat. No. 5,768,420. In this known method, curve recognition is described by means of a ratio that is named “ratio of tangents”. A curve, for instance, a portion of a character is mapped by selecting a sequence of pairs of points along the curve, where the tangents in the two points of each pair intersect at a certain angle. The ratio between the distances from the intersection point to the respective points of the pair is calculated and makes up an identification of the curve. This method is in principle not sensitive to translation, scaling and rotation. However, it is limited in many respects. Above all, it does not allow certain curve shapes in which there are not two points whose tangents intersect at the determined angle. It is common that at least portions of a character comprise such indeterminable curve shapes for a selected intersection angle. This reduces the reliability of the method.